Traction devices providing a strong force over a short distance are either electrically, hydraulically or mechanically operated. Devices using an electric motor to provide traction are relatively expensive. The same is true of hydraulically-operated devices, as close-fitting moving parts and pressure seals are necessary. Mechanical devices, using springs or gravity, are cheap to install but have the disadvantage that they represent an additional load which has to be overcome at times when traction is not required.